Human topoisomerase I is a 92 kDa enzyme that helps to control the level of DNA supercoiling in cells. It is also found to be the sole target of camptothecin, an extremely promising anti-cancer drug currently in clinical trials. Elucidating the three-dimensional structure of topoisomerase I bound to DNA would allow us to unravel the fascinating mechanism of this enzyme, and would facilitate the design of improved anti-cancer drugs. We are currently well on our way to determining the crystal structure of human topoisomerase I bound to DNA. Synchrotron radiation has played a critical role in the structure determination of this important protein-DNA complex.